A shortening of processing time is greatly desired in the photographic industry today and there is an urgent need for the development of silver halide photographic materials which are suitable for rapid processing.
The water solubility of silver halide is increased when the silver chloride content is increased and shorter developing and fixing times can be achieved, and silver halides which are suitable for rapid processing have been obtained in this way.
Silver halide grains which have a high silver chloride content (referred to herein as "high silver chloride grains") generally have a cubic form consisting of (100) planes, and it is desired to obtain grains which have a form other than a cubic form, such as a tabular form or a regular crystalline form, i.e., an octahedral or tetradecahedral form, which has (111) planes.
It is well known to those in the industry that tabular grains in which the diameter is considerably larger than the thickness are preferred for raising the speed of a silver halide emulsion for photographic purposes, increasing sharpness, and improving graininess, color sensitizing efficiency with sensitizing dyes and covering power. The only known tabular grains which have a high silver chloride content in excess of 50 mol % and which do not contain bromide or iodide inside are those formed by the method of U.S. Pat. No. 4,399,215 in which the grains are formed at a pAg within the range from 6.5 to 10 and a pH maintained within the range from 8 to 10 using ammonia; those formed by the method of U.S. Pat. No. 4,400,463 in which grain formation is carried out in the presence of aminoazaindene and a peptizer which has thioether bonds; and those formed by the method of JP-A-62-218959 in which thiourea based compounds are used (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application").
However, with the method in which ammonia is used it is difficult to form emulsions generally used for light-sensitive materials for rapid processing in which the volume of the grains is comparatively small (i.e., not more than 1 .mu.m.sup.3) in order to further increase the solubility of the highly soluble high silver chloride content grains. Further, because of the inevitably high pH during the formation of the grains, increased fogging often occurs with sensitive high silver chloride content emulsions. Therefore, the conditions under which the grains can be formed by using this method are greatly restricted.
The peptizers in the methods in which peptizers which have thioether bonds are used are synthetic polymers. It is difficult to obtain copolymers with good reproducibility, the polymerization initiator may contain impurities which are harmful photographically, and there is a further disadvantage in that the desalting process may be complicated. Furthermore, it is costly to eliminate these difficulties and this is disadvantageous from the industrial point of view.
On the basis of the facts outlined above it is clear that the development of a method for obtaining high silver chloride content tabular grains with good reproducibility in the acid-neutral region using cheap, low molecular weight compounds which are easily synthesized and refined, either alone or in conjunction with gelatin which is the normally used as a general purpose peptizer, is clearly desirable.
The above-mentioned tabular grains are grains which have twinned crystal planes within the grain and in which the outer surfaces (i.e., basal planes) are (111) planes, and few methods are known for the preparation of high silver chloride content grains which have no twinned crystal planes and which are regular crystals, consisting of octahedra or tetradecahedra which have (111) planes as outer surfaces.
Such methods include those described by Claeo et al., The Journal of Photographic Science, Volume 21, 39 (1973) and Wyrsch, International Congress of Photographic Science III-13, 122 (1978).
The compounds dimethyl thiourea, thiourea and adenine are used by Claeo et al. but the photographic properties of the octahedral grains obtained are not fully reported. Moreover, when considered from the point of view of the compound structure it can be concluded that they are compounds which, like adenine, are quite strongly adsorbed on silver halides and compounds which have unstable sulfur atoms which readily give rise to fogging.
In Wyrsch, ammonia and a large amount of cadmium nitrate are used, octahedral silver chloride grains are obtained and a photographic performance similar to that of cubic grains is obtained, but cadmium is undesirable for use in practice from the point of view of pollution. Furthermore, high silver chloride content grains are easily fogged, so the use of ammonia is undesirable, and the preparation of high silver chloride content octahedral grains without the use of ammonia and without pollution problems is desirable.
Thus, as described above, the development of a novel method for the preparation of regular crystalline grains, which is to say tetradecahedral or octahedral grains, or tabular grains which have twinned crystal planes within the grains, with fresh and stable (111) planes on the outer surface is clearly desirable.